Weathering Processes

Weathering

1. Weathering vs. Erosion

2. Joints: Setting the Stage

3. Physical (Mechanical) Weathering

4. Chemical Weathering

1. Weathering vs. Erosion

Common Error Made in K-12 Earth

Science Teaching: these are synonyms

No!

Weathering – chemically dissolving candy or physically crunching candy – breakdown in place

Erosion – moving pieces (dissolved or as fragments)

1st - Weathering = decay in place

2nd - Detach = break off

3rd Erode = move

1st – decayed

2nd – detached

3rd – eroded

s

2. Joints: Setting the StageJoints – fractures (that allow water to

penetrate and weather the rock)

Many ways to make joints:

Cooling & contraction Tectonic Stresses

Very different!

Columnar Jointing

From contraction

after lava flow cools

or faulting/folding

stresses the rock

Joints can even fracture sedimentary strata from regional tectonic pressures

3. Physical (Mechanical) Weathering

• Frost weathering

• Pressure release weathering

• Salt Weathering

• Thermal Expansion/Contraction

• Wedging (Fissuresols & Calcrete)

• Wetting/Drying

• Root pressure

Frost Weathering

Breaks rock along small fractures

Breaks rock along large fractures& produces jagged alpine topography

Aesthetic, so used in commercials

Pressure Release

Sometimes called “exfoliation” (like exfoliating skin) – incorrect usage

pressure release shells

Erosionremovesoverburden &shells pop offas pressure isreleased

Glacial Erosion Great Way to Generate Pressure Release

Salt Weathering

Salt is common in deserts

Salt is common along coasts

Mechanics of Salt Weathering• Salt Crystal Growth: Extreme

pressures in cracks and rock pores are

caused by salt crystal growth from

solution. There are varying causes of

and extents to which salt growth occurs.

• Hydration: The hydration of various salts

causes expansion & contraction, pushing

apart the silicate host minerals

• Thermal Expansion:

During temperature

fluctuations, salts

trapped in pores may

expand to a greater

degree than the

surrounding rock

minerals.

Caverns (tafoni)

Base of rock notching (basal weathering)

Wedging

Caverns (tafoni – larger

cavernous forms)

(Goudie and Viles, 1997:168)

Caverns (alveoli – smaller cavernous forms)

Weathering along bases of rocks

Wedging

Sequence in Rock Fractures

Generalized Process

Fissuresol wedging can pry apart bedrock

Fissuresol wedging can pry apart

mountain sides and

giant boulders

Thermal Expansion/Contraction

• Thermal Expansion/Contraction

A bit of moisture & sudden heat makes the rock pop

Vermillion Cliffs

Wetting/Drying

Root Pressure

Online Animations

Physical Weathering Visualizations

http://serc.carleton.edu/NAGTWorkshops/visualization/collections/physical_weathering.html

Physical weathering “sets up” chemical weathering

Why do you ground coffee?to increase surface area

4. Chemical WeatheringEgyptian Obelisk – chemically weathered

when brought to wetter environment, so water matters!

DissolutionSugar & Salt Dissolves – so do rocks

Best example: limestone

Other rocks dissolve too, but slower than limestone

Acid Rain Accelerates Decay

Crosses political boundaries …

Taj Mahal Athens

Oxidation

Hydrolysis

Water molecules at the mineral surface dissociate into H+ and OH- and the mobile H+ ions penetrate the crystal lattice, creating a charge imbalance, that causes cations (important nutrients) such as Ca2+ , Mg2+, K+ and Na+ to diffuse out. For example, the feldspar reacts to decay and leaves a residue of clay mineral.

H+

OH-

H+

Hydration

Water alters structure

Complexation Metals released from primary minerals such as iron and manganese build complexes with organic components, such as fulvic acids and humic acids, causing an imbalance between cations and anions – that leads to mineral decay

Online Animations

Chemical Weathering Visualizations

http://serc.carleton.edu/NAGTWorkshops/visualization/collections/chemical_weathering.html